This report on the third series of coal-dust explosibility tests at the experimental mine near Bruceton, Pa., about 12 miles from the Pittsburgh experiment station of the bureau, covers the years 1919 to 1924, inclusive. SUSPENSION OF TESTS: The previous testing of dusts to determine their explosibility was suspended in 1918, during the World War, in order to allow the bureau to take up a series of problems dealing with military mining, sound ranging, and tests of explosives, including experimental work with liquid-oxygen explosives. Another problem investigated was the possibility of storing helium gas under high pressure, up to 100 atmospheres, in mine chambers, lined and unlined, instead of in metal bottles. To make such chambers gas-tight, even when lined with copper and with lead sheets, proved difficult because of the elasticity of the walls. After the war the mine had to be used for special purposes. The most important purpose was to test the effectiveness and limitations of a novel method of ventilating an underground tunnel in which internal-combustion motors-automobiles and trucks-are used. The immediate application of the results was to be in the proposed Holland vehicular tunnels under the Hudson River, and the study was undertaken at the request of State officials of New Jersey and New York. The investigation obtained fundamental data on the amount of poisonous gases given off by automobiles and trucks, the physiological effects of those gases, the necessary limits for ventilation, and the adequacy of the proposed system of ventilation and its possible application in mining. A report was rendered to the New York-New Jersey Bridge and Tunnel Commission. Then, to take advantage of the set-up of delicate gauges for recording air flow and pressure, a series of experiments was made in order to determine frictional losses in the ventilation of coal mines, under various conditions and arrangements. Another phase of the investigation started in connection with the tunnel work was a study of the flow of heat from the natural walls, roof, sides, and floor into the air current and vice versa, a problem of much importance in ventilation of deep or hot mines. A report on this investigation is to be published. Thus, the testing of coal dust was again suspended in 1921 to 1923. Meanwhile, the number of explosion disasters in coal mines in this country had steadily diminished; in 1918 explosions killed 41 men, in 1919 they killed 81, in 1920 but 47, and in 1921 only 16, as contrasted with about 2,000 men killed each year in coal mines by falls of roof, haulage, and other underground accidents. This reduction in the number of men killed by explosion disasters as compared with 882 killed from this cause in a single year (1907) seemed to indicate that the danger of these disasters had been overcome. Apparently, however, this reduction was a matter of coincidence. Disastrous coal-dust explosions began to recur in the bituminous mines of the country. In 1921 explosions killed 264 men, in 1923 they killed 286, and in 1924, 445, the largest number killed in coal-dust explosions since 1907. RESUMPTION OF ACTIVITIES: This increase naturally aroused great anxiety in the coal-mining industry. It raised the question why the recommendations of the bureau for rock dusting had not been accepted and followed in this country as similar recommendations by Government agencies in Great Britain and in France had been followed in those countries. In Great Britain the recommendations were made mandatory; in France most of the mines adopted rock dusting after the Government had approved it, and Government pressure was being put on the others. In 1923 the senior author of this bulletin was sent abroad to examine the methods of rock dusting used in Great Britain, France, and Germany. His report was issued as Bulletin 225. Meanwhile, testing of coal dusts was resumed at the experimental mine, especially tests for determining the relative explosion hazards of coal dusts from various typical mines by the procedure laid down in Bulletin 167. Later the Bureau of Mines, at the request of the British Mines Department, entered into a cooperative agreement with the British Safety in Mines Research Board for joint research in mine safety, including coal-dust explosion investigations. The British Government sent Dr. R. V. Wheeler, director of research, his assistant, W. R. Chapman, and Henry Walker, then deputy chief, and later H. M. chief inspector of mines, to the United States in the spring of 1924 to arrange with the bureau officials a joint program of work. Later the director of the Bureau of Mines, H. Foster Bain, visited England on the same mission. While here the British representatives witnessed tests of the Silkstone (Yorkshire) coal dust, used at the Eskmeals station as the British standard dust for testing. They also greatly advanced the cause of rock dusting in the bituminous mines of this country by attending numerous meetings of mining men, where they explained the success that had followed compulsory rock dusting or "stone" dusting in Great Britain. The Bureau of Mines sent the junior author, H. P. Greenwald, assistant physicist, to the British testing station at Eskmeals, where Doctor Wheeler placed him in charge of a group of coal-dust tests of English coals in the main explosion gallery. While there he conducted tests to compare the Pittsburgh standard dust used at the bureau's experimental mine with the Silkstone dust used as standard at Eskmeals. The results of these comparative tests showed that the British standard coal dust behaves essentially like the American standard dust from the experimental mine, thus confirming results of similar comparative tests made in that mine. It is expected that the agreement with the British station, which provides for interchange of personnel, materials, and instruments, and for prompt interchange of information on the progress of the different lines of research, will be stimulating to both sides, prevent unnecessary duplication, provide standards for comparison, and greatly advance knowledge of preventive measures. FACTORS DEVELOPED BY NEW SERIES OF TESTS: The series of explosion tests covered by this bulletin has not developed any strikingly new fundamental facts. The results of the tests largely confirm those reported in Bulletin 167, but more precise information has been obtained on the underlying factors that govern coal-dust; explosions, particularly in regard to the effect of the size of dust particles on their relative explosibility in air and the great influence of fire damp in increasing the explosibility of dust mixtures. Al though fire damp increases the sensitiveness of all coal dusts to ignition and propagation, it has the most marked effect on the low-volatile coal dusts and on coarse coal dusts, which are not so explosive when it is absent. Some additional data were obtained on the effect in the starting and propagation of an explosion, of release of pressure, both near the point of origin and at a distance along the path, but much remains to be done in studying the mechanism of explosions. The wide adoption of the rock-dusting method by American mine operators since the campaign inaugurated by the bureau in 1923 has led to a stream of inquiries from mining men, State officials, and liability insurance rating bureaus regarding rock dust and its application. Accordingly, the bureau issued Serial 2606,6 containing the specifications for rock dusting. Appended to this bulletin is the schedule on rock-dusting practice approved by the American Engineering Standards Committee; it accords with the specifications in Serial 2606. Special conditions in various coal fields call for special rock-dust treatment, and a need has developed for rock-dust barriers where generalized dusting is considered impracticable. The staff of the experimental mine is now engaged in carrying on special experiments and tests of barriers with a view to formulating specifications for "recommended" barriers. Since the issuance of Bulletin 167 the Government has acquired the coal and surface contiguous to the mine, thus assuring the permanence of the mine and securing from loss the large investment in the mine and the permanent apparatus that would be practically valueless if the mine were not retained. Although some extensions were made within the experimental mine, chiefly to provide for other testing, the standard arrangements for explosibility testing have not been changed.